This invention relates to naphtha fuels useable in Compression Ignition (CI) combustion engines as well as to a process for production of such naphtha fuels. More particularly, this invention relates to naphtha fuels produced from a mainly paraffinic synthetic crude which is produced by the reaction of CO and H2, typically by the Fischer-Tropsch (FT) process.
Products of a FT hydrocarbon synthesis process, particularly the products of a cobalt and/or iron based catalytic process, contain a high proportion of normal paraffins. Primary FT products provide notoriously poor cold flow properties, making such products difficult to use where cold flow properties are vital, e.g. diesel fuels, lube oil bases and jet fuel. It is known in the art that octane number and cetane number are normally inversely related i.e. a higher octane number is typically associated with a lower cetane number. It is also known that naphtha fractions intrinsically have low cold flow characteristics like congealing and cloud points. There is thus an incentive for a process to produce a synthetic naphtha fuel obtained from the FT process which has good cold flow characteristics and a Cetane number compatible with CI engine fuel requirements. Additionally, such synthetic naphtha fuel may have acceptable biodegradability properties.
The synthetic naphtha fuel described in this invention is produced from a paraffinic synthetic crude (syncrude) obtained from synthesis gas (syngas) through a reaction like the FT reaction. The FT primary products cover a broad range of hydrocarbons from methane to species with molecular masses above 1400; including mainly paraffinic hydrocarbons and smaller quantities of other species such as olefins, and oxygenates.
The prior art teaches in U.S. Pat. No. 5,378,348 that by hydrotreating and isomerizing the products from a Fisher-Tropsch reactor one can obtain a jet fuel with freezing point of xe2x88x9234xc2x0 C. or lower due to the isoparaffinic nature of this fuel. This increased product branching relative to the waxy paraffin feed corresponds with a Cetane rating (combustion) value less than that for normal (linear) paraffins, depicting that an increase in branching reduces the Cetane value of paraffinic hydrocarbon fuels.
Surprisingly, it has now been found by the applicant, that a hydroprocessed synthetic naphtha fuel may be produced having a Cetane number, typically in excess of 30, as well as good cold flow properties. The synthetic naphtha fuels of the present invention could be used on their own or in blends in CI engines, typically where diesel fuels are presently used. This would lead to the more stringent fuel quality and emission specifications being satisfied. The synthetic naphtha fuels of the present invention may be blended with conventional diesel fuels to have lower emissions, good cold flow characteristics, low aromatics content and acceptable cetane numbers.
Thus, according to a first aspect of the invention, there is provided a process for the production of a synthetic naphtha fuel suitable for use in CI engines, the process including at least the steps of:
a) hydrotreating at least a fraction of a Fischer-Tropsch (FT) synthesis reaction product of CO and H2, or a derivative thereof;
b) hydrocracking at least a fraction of the FT synthesis product or a derivative thereof, and
c) fractionating the process products to obtain a desired synthetic naphtha fuel characteristic.
The process may include the additional step of blending the fractionated process products in a desired ratio to obtain a synthetic naphtha fuel having desired characteristics for use in a CI engine.
The process as described above may produce a synthetic naphtha wherein some of the desired characteristics include:
having a high Cetane number in excess of 30;
having a low sulfur content below about 5 ppm;
having good cold flow properties; and
having more than 30% isoparaffins, wherein the isoparaffins include methyl and/or ethyl branched isoparaffins.
According to yet another aspect of the invention, there is provided a process for producing a synthetic naphtha fuel having a Cetane number higher than 30, the process including:
(a) separating the products obtained from synthesis gas via the FT synthesis reaction into one or more heavier fraction and one or more lighter fraction;
(b) catalytically processing the heavier fraction under conditions which yield predominantly distillates;
(c) separating a naphtha product fraction of step (b) from a heavier product fraction which is also produced in step (b); and
(d) optionally, blending the naphtha product obtained in step (c) with at least a portion of the one or more lighter fraction of step (a), or products thereof.
The catalytic processing of step (b) may be a hydroprocessing step, for example, hydrocracking or mild hydrocracking.
The process for producing a synthetic naphtha fuel may include one or more additional step of fractionating at least some of the one or more lighter fraction of step (a), or products thereof, prior to step (d).
The process for producing a synthetic naphtha fuel may include the additional step of hydrotreating at least some of the one or more light fraction of step (a), or products thereof, prior to step (d).
The one or more heavier fraction of step (a) may have a true boiling point (TBP) in the range of about 70xc2x0 C. to 700xc2x0 C., however, it may be in the range 80xc2x0 C. to 650xc2x0 C.
The one or more lighter fraction may have a true boiling point (TBP) in the range xe2x88x9270xc2x0 C. to 350xc2x0 C., typically in the range xe2x88x9210xc2x0 C. to 340xc2x0 C.
The product of step (d) may boil in the range 30xc2x0 C. to 200xc2x0 C. The product of step (d) may boil in the range 40xc2x0 C. to 155xc2x0 C., as measure by the ASTM D86 method.
The product of step (d) may be a naphtha fuel.
The product of step (d) may have a Cloud Point below xe2x88x9230xc2x0 C., typically xe2x88x9240xc2x0 C and even below xe2x88x9250xc2x0 C.
The product of step (d) may be obtained by mixing the naphtha product fraction obtained in step (c) with at least a portion of the one or more lighter fraction of step (a), or products thereof, in a volume ratio of between 1:24 and 9:1, typically 2:1 and 6:1, and in one embodiment, in a volume ratio of 50:50.
The invention extends further to a process for the production of synthetic naphtha fuels suitable for CI engines, from FT primary products, comprising predominantly short chain linear and branched paraffins.
In this process, the waxy product from the FT process is separated into at least two fractions, a heavier and at least one lighter fraction. The lighter fraction may be subjected to mild catalytic hydrogenation to remove hetero-atomic compounds such as oxygen and to saturate olefins, thereby producing material useful as naphtha, diesel, solvents, and/or blending components therefor. The heavier fraction may be catalytically hydroprocessed without prior hydrotreating to produce products with good cold flow characteristics. This hydroprocessed heavier fraction could be blended with all or part of the hydrogenated and/or unhydrogenated light fraction to obtain, after fractionation, naphtha fuel characterised by an acceptable Cetane number.
The catalysts suitable for the hydroprocessing steps are commercially available and can be selected towards an improved quality of the desired final product.
According to a further aspect of the invention, there is provided a synthetic naphtha fuel having a Cetane number above 30 and a Cloud Point below xe2x88x9230xc2x0 C., said naphtha fuel having an isoparaffinic content substantially as described above.
In one embodiment, the synthetic naphtha fuel is a FT product.
The invention extends to a fuel composition including from 10% to 100% of a synthetic naphtha fuel as described above.
Typically, the fuel composition may include from 0 to 90% of one or more diesel fuels.
The fuel composition may include at least 20% of the synthetic naphtha fuel, the composition having a Cetane number greater than 40 and a Cloud Point below 2xc2x0 C. Using the synthetic naphtha as Cloud Point depressor may result in at least 2xc2x0 C. depression in Cloud Point of the fuel composition.
The fuel composition may include at least 30% of the synthetic naphtha fuel, the composition having a Cetane number greater than 40 and a Cloud Point below 0xc2x0 C. Using the synthetic naphtha as Cloud Point depressor may result in at least 3xc2x0 C. depression in Cloud Point for the fuel composition.
The fuel composition may include at least 50% of the synthetic naphtha fuel, the composition having a Cetane number greater than 40 and a Cloud Point below 0xc2x0 C., more typically below xe2x88x924xc2x0 C. Using the synthetic naphtha as Cloud Point depressor may result in at least 4xc2x0 C. depression in Cloud Point for the fuel composition, or more typically at least 8xc2x0 C. depression.
The fuel composition may include at least 70% of the synthetic naphtha fuel, the composition having a Cetane number greater than 40 and a Cloud Point below xe2x88x9210xc2x0 C., more typically below xe2x88x9215xc2x0 C. Using the synthetic naphtha as Cloud Point depressor may result in at least 13xc2x0 C. depression in Cloud Point for the fuel composition, or more typically at least 18xc2x0 C. depression.
The blend composition may further include from 0 to 10% additives to improve other fuel characteristics.
The additives may include a lubricity improver. The lubricity improver may comprise from 0 to 0.5% of the composition, typically from 0.00001% to 0.05% of the composition. In some embodiments, the lubricity improver comprises from 0.008% to 0.02% of the composition.
The fuel composition may include, as the diesel, a crude oil derived diesel, such as US 2-D grade (low sulphur No. 2-D grade for diesel fuel oil as specified in ASTM D 975-94) and/or CARB (California Air Resources Board 1993 specification) diesel fuel, and/or a South African specification commercial diesel fuel.